Inverse Synthetic Aperture Radar (ISAR) is a radar signal processing technique used to form images of moving targets. The ISAR may be mounted on a movable platform such as an airplane or ship. For example, an airplane may use an ISAR to produce radar images of a speedboat. A two-dimensional image can be formed by separating radar returns from an object in range and frequency. Different points along a rotating object will have different line of sight velocities in the direction of the radar sensor thus giving different Doppler shifts. If no corrections are made, nonlinear Doppler shifts can often result in defocused images. Likewise, uncorrected range variations can cause blurred images.
Radar imaging systems generally use a tracker to maintain a target within the radar range gate. The tracker locates a high intensity point on the target to track, determines the range to the target, and controls the radar pulse timing. One of the challenges in ISAR is keeping a track on the moving target in real time in order to keep radar returns from each point on the object in the same radar sensor range bin over the data collection interval. For very high resolution radar systems, the challenge becomes even more difficult.
With ISAR, imaging of dynamic targets causes range tracker jitter (uncompensated translational motion) and range migration (rotational motion) that results in unfocused images with poor image quality. If digital pulse compression is used, range tracker jitter can induce a large amount of phase noise in the target radar returns resulting in poor image quality.
Due to the aforementioned problems, conventional ISAR systems are inadequate to generate high quality ultra-high resolution radar images.